Use a plastic or styrofoam cup and place it face down with the mouth of the cup over the drain. As the water starts to fill up it will create a vacuum and seal the drain opening.
The rubber end of the plunger will definitely create a water tight seal but unless you have a brand new, never been used plunger then then this is not a good idea.
An empty single-serve pod seems like it would be the ideal size and shape to use as a one-time solution for a bath plug, unfortunately if the Keurig cup is used it will have a small hole in the bottom that will allow the water to drain through.
A plastic lid seems like it would be one of the better alternative bath plugs but unless you have something weighing it down to create a seal it will just float up once you start adding water.